1. Field of the Invention
The general field of the invention is that of anti-collision systems for aircraft and more particularly that of the presentation of anti-collision information.
2. Description of the Prior Art
For an aircraft in flight, it is fundamental to have very precise knowledge of other aircraft situated in its immediate environment so as to avoid any risk of collision. This problem is particularly crucial in a certain number of applications where aircraft are obliged to fly at low altitude with visibility conditions which may be reduced. Historically, from the years 1960-1970 onwards, a solution independent of air traffic control has gradually emerged. This solution is known by the name of TCAS, the acronym standing for “Traffic alert and Collision Avoidance System”.
Today, several families of TCAS have been developed and are used:                The first generation termed TCAS I provides only alerts of “Traffic Advisory” or TA type regarding proximity of intruders, vehicles or other aircraft presenting a risk for the aircraft. TCAS I is used essentially in general aviation, that is to say the field of small aeroplanes.        TCAS II provides on the one hand alerts of TA type regarding intruder proximity and on the other hand resolutions of conflicts by suggesting avoidance manoeuvres to the pilot. The operating mode is called RA for “Resolution Advisory”. These avoidance manoeuvres are performed in a vertical plane by the craft climbing or descending. It is used essentially in commercial aviation. TCAS II was made compulsory on all airliners in the 1990s.        TCAS III, still under development, is an improvement to TCAS II additionally allowing resolution of the conflicts in RA mode in the horizontal plane through left or right turning manoeuvres.        
The TCAS information can be presented to the pilot in various ways. As an example, FIG. 1 shows the presentation of the intruders on a screen of ND (Navigation Display) type. The intruders are presented in a 2D horizontal plane in relation to the aircraft 100 in so-called “ROSE” mode, for compass rose. The aircraft 100 occupies the centre of the “ROSE” 101 represented by a graduated circle. The shape and the colour of the intruders differ depending on their associated degree of danger and depending on the operating world of the TCAS.
By way of examples, the aircraft 102 is close, at a relative altitude of 1100 feet below the aircraft 100, relative altitude symbolized by the indication “−11”. This aircraft is climbing, symbolized by an upward arrow in FIG. 1. It is depicted by a solid white or cyan diamond representing a threat in so-called PT mode, standing for “Proximate Traffic”. According to aeronautical conventions, when the diamond is solid, the threat is of PT type, if the diamond is blank, then the threat is of OT type, signifying “Other Traffic”.
The aircraft 103 is a threat in RA mode, standing for “Resolution Advisory”. It is situated at a relative altitude of 100 feet below the aircraft 100 and climbing. The colour of the square which represents it is red.
The aircraft 104 is an intruder in TA mode, standing for “Traffic Advisory”; it is 900 feet above the aircraft 100 and descending. The colour of the circle which represents it is amber.
As may be seen, the interpretation of the information by the pilot is far from being immediate, and this may turn out to be particularly dangerous in the case of imminent risk of collision.
The new Synthetic Vision Systems (SVS) currently afford pilots a synthetic representation of the outside world and therefore better awareness of the surrounding dangers such as collisions with the ground without loss of control, commonly called CFIT (Controlled Flight Into Terrain). These SVS systems currently make it possible to display in 3D a synthetic terrain as well as natural or artificial obstacles (tall buildings, etc.) An improvement to the presentation of the information provided by TCAS has been proposed in the Honeywell patent application entitled “Perspective View Conformal Traffic Target Display” published under the international number WO2007/002917A1which corresponds to U.S. application Ser. No. 11/170,356, filed Jun. 29, 2005, now U.S. Pat. No. 7,286,062. FIG. 2 shows an exemplary presentation of the intruders on a screen 200 of PFD type for “Primary Flight Display” according to the provisions of this patent application. The intruders are presented in 3D in a conformal manner, that is to say positioned at their real location in the landscape. Complementary information is associated with the intruders to aid the pilot to pinpoint their position, above or below a reference altitude and their degree of separation obtained by virtue of a variation in the dimension of the symbols. FIG. 2 presents the air traffic in a 3D conformal synthetic view of the terrain 201. This view also comprises a representation 210 of the PFD information. The intruders are presented in the forward sector of the aeroplane. The intruders 204 and 205 are represented by larger or smaller squares according to their distance relative to the aeroplane. Other symbols are added to aid the pilot to interpret the relative altitude of the intruder with respect to the aeroplane. Thus, the symbols 202 and 203 representing vertical masts give the position and the height of the intruders above the ground. This presentation is well suited to airliners which fly at relatively high altitude.
Although the new SVS systems enable the pilot to be better apprised of the situation of the intruders, in particular their type, their positioning, their behaviour, their performance etc, these new systems are insufficient to accomplish low-altitude missions.
Specifically, intruders are very rare for airliners flying by instruments, which follow pre-established flight plans in strict air corridors and are controlled from the ground with radars by the air traffic bodies. On the other hand, helicopters or small aeroplanes can fly in number at low altitude, for example, to help to rescue a large number of victims, within the framework of a “red” plan or within the framework of civil protection missions.
In this case, flight is essentially by sight, without any established flight plan and/or outside of conventional radar coverage. The visibility conditions may deteriorate in the case of night flights, dazzle from the sun, the presence of smoke in fire-fighting missions, etc. The aircraft also have more dynamic and more varied trajectories (turns, climbs, descents, etc) than those of airliners. Under these conditions, it is particularly important to properly apprise the pilot as to where the intruders are with respect to him and essentially as to their performance such as, for example, their closing speed or their danger rating.